[RFC PATCHv3 2/2] block: fix nr_hw_queue update racing with disk addition/removal

[Nilay Shroff <nilay@linux.ibm.com>]

The nr_hw_queue update could potentially race with disk addtion/removal
while registering/unregistering hctx sysfs files. The __blk_mq_update_
nr_hw_queues() runs with q->tag_list_lock held and so to avoid it racing
with disk addition/removal we should acquire q->tag_list_lock while
registering/unregistering hctx sysfs files.

With this patch, blk_mq_sysfs_register() (called during disk addition)
and blk_mq_sysfs_unregister() (called during disk removal) now runs
with q->tag_list_lock held so that it avoids racing with __blk_mq_update
_nr_hw_queues().

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 block/blk-mq-sysfs.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 6113328abd70..3feeeccf8a99 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -225,25 +225,25 @@ int blk_mq_sysfs_register(struct gendisk *disk)
 
 	ret = kobject_add(q->mq_kobj, &disk_to_dev(disk)->kobj, "mq");
 	if (ret < 0)
-		goto out;
+		return ret;
 
 	kobject_uevent(q->mq_kobj, KOBJ_ADD);
 
+	mutex_lock(&q->tag_set->tag_list_lock);
 	queue_for_each_hw_ctx(q, hctx, i) {
 		ret = blk_mq_register_hctx(hctx);
 		if (ret)
-			goto unreg;
+			goto out_unreg;
 	}
+	mutex_unlock(&q->tag_set->tag_list_lock);
+	return 0;
 
-
-out:
-	return ret;
-
-unreg:
+out_unreg:
 	queue_for_each_hw_ctx(q, hctx, j) {
 		if (j < i)
 			blk_mq_unregister_hctx(hctx);
 	}
+	mutex_unlock(&q->tag_set->tag_list_lock);
 
 	kobject_uevent(q->mq_kobj, KOBJ_REMOVE);
 	kobject_del(q->mq_kobj);
@@ -256,9 +256,10 @@ void blk_mq_sysfs_unregister(struct gendisk *disk)
 	struct blk_mq_hw_ctx *hctx;
 	unsigned long i;
 
-
+	mutex_lock(&q->tag_set->tag_list_lock);
 	queue_for_each_hw_ctx(q, hctx, i)
 		blk_mq_unregister_hctx(hctx);
+	mutex_unlock(&q->tag_set->tag_list_lock);
 
 	kobject_uevent(q->mq_kobj, KOBJ_REMOVE);
 	kobject_del(q->mq_kobj);
-- 
2.47.1